Method for making low core loss, well-bonded, soft magnetic parts

ABSTRACT

A ferromagnetic powder comprising ferromagnetic particles coated with a material that does not degrade at temperatures above 150° C. and permits adjacent particles to strongly bind together after compaction such that parts made from the ferromagnetic powder have a transverse rupture strength of about 8,000 to about 20,000 pounds/square inch before sintering, The coating includes from 2 to 4 parts of an oxide and one part of a chromate, molybdate, oxalate, phosphate, or tungstate. The coating may be substantially free of organic materials. The invention also includes a method of making the ferromagnetic powder, a method of making soft magnetic parts from the ferromagnetic powder, and soft magnetic parts made from the ferromagnetic powder.

This is a Divisional of U.S. patent application Ser. No. 09/183,080filed Oct. 30, 1998; which in turn is a Divisional of U.S. patentapplication Ser. No. 09/010,073 filed Jan. 21, 1998 now U.S. Pat. No.5,982,073 which claims priority from Provisional Application No.60/069,832, filed Dec. 16, 1997.

FIELD OF THE INVENTION

This invention relates to ferromagnetic powder intended for use in themanufacture of both soft and hard (permanent) magnetic parts. Theinvention further relates to a method of making such ferromagneticpowder, methods of making parts from the ferromagnetic powder and toparts, including stators, rotors, armatures and actuators made from theferromagnetic powder.

BACKGROUND OF THE INVENTION

Magnetic materials generally fall into two classes, magnetically hardsubstances which may be permanently magnetized, and soft magneticmaterials whose magnetization may be reversed at relatively low appliedfields. Permeability and coercive filed values are a measurements of theease with which a magnetic substance can be magnetized or carry amagnetic flux. Permeability is indicated by the ratio of B/H. Thecoercive force, H_(c), is the magnetic force or field intensitynecessary to change magnetic induction B from—to +. It is important insoft magnetic materials that energy loss, normally “core loss” is keptto a minimum whereas in hard magnetic materials it is preferred toresist changes in magnetization. High core losses are thereforecharacteristic of permanent magnetic materials and are undesirable insoft magnetic materials.

Soft magnetic core components are frequently used in electrical/magneticconversion devices such as motors, generators and transformers andalternators, particularly those found in automobile engines. The mostimportant characteristics of ferromagnetic soft magnetic core componentsare their maximum induction, magnetic permeability, and core losscharacteristics. When a magnetic material is exposed to a rapidlyvarying magnetic field, a resultant energy loss in the core materialoccurs. These core losses are commonly divided into two principlecontributing phenomena: hysteresis and eddy current losses. Hysteresisloss results from the expenditure of energy to overcome the retainedmagnetic forces within the iron core component. Eddy current losses arebrought about by the production of induced currents in the iron corecomponent due to the changing flux caused by alternating current (AC)conditions.

Conventional practice has been to fabricate soft magnetic materials andparts by forming laminated structures of thin die stamped ferroussheets, typically a silicon-iron alloy. The sheets are oriented parallelto the magnetic field to assure low reluctance. The laminations must bestacked in correct alignment and the stack of laminations must then besecured together, for example, by welding, riveting, gluing, etc. Thesheets may be varnished phosphated or otherwise coated to provide forsome insulation between them. This insulation is intended to preventcurrent from circulating between sheets and therefore to keep eddycurrent losses low. In die stamping, there is however, a certain amountof scrap loss and hence unnecessary expense. In addition, the stampingprocess sometimes results in burrs requiring a subsequent deburring stepand a thick bed coating to keep the sharp edges from cutting theinsulation on the electrical conductors. Moreover, the stacked cores areknown to suffer from large core losses at higher frequencies and areacoustically noisy (hysteresis) since the laminations tend to vibrate.This vibration also contributes to energy loss. U.S. Pat. No. 3,670,407to Mewhinney et al. describes a stator made by such a stacked laminationand an attempt to reduce the eddy currents therein.

Another significant drawback to making soft magnetic parts from steellaminate structures is that it is difficult and time consuming to makeparts having a three-dimensional configuration for moving flux out ofthe plane of the lamination. Certain three-dimensional configurationsare very difficult and expensive to achieve with steel laminatestructures.

The use of powdered metals avoids the manufacturing burden inherent inlaminated structures and provides for a wider variation in the shape ofthe part. These materials made from consolidated powdered metals havehowever generally been limited to being used in applications involvingdirect currents. Direct current applications, unlike alternating currentapplications, do not require that the iron particles be insulated fromone another in order to reduce eddy currents. Hence, various attemptshave been made in the past to form magnetic materials from powdershaving the desired characteristics necessary for expanded applicationsincluding alternating current. For example, U.S. Pat. No. 3,245,841 toClarke et al. describes a process for producing steel powder by treatingthe powder with phosphoric acid and chromic acid to provide a surfacecoating on the steel particles of iron phosphate and chromium compounds.This process however results in poorly bonded material with relativelypoor insulating properties. The use of hexavalent chromium in theseprocesses posses a significant health risk since it is carcinogenic.Hence, expensive waste treatment systems must also be employed.

In U.S. Pat. No. 4,602,957 to Pollock, et al., iron powders are treatedwith oxidizing agents such as potassium or sodium dichromate prior tocompaction. The compact is then partially sintered at 600° C. Thesepartially sintered compacts are reported to have increased resistivityand decreased hysteresis losses when compared to bulk iron compacts. Thestep of sintering the part following compaction, is however, necessaryto achieve satisfactory mechanical properties in the part by providingparticle to particle bonding and hence strength. However, sinteringincreases manufacturing complexity and adds to the cost of the finishedpowder metallurgy part. In addition, sintering causes volume changes andresults in a manufacturing process with poor dimensional control.

In other known processes to minimize eddy current losses in ferrousparts made by powder metallurgy, soft iron particles are coated withthermoplastic materials before pressing. U.S. Pat. No. 4,947,065 to Wardet al. and U.S. Pat. No. 5,198,137 to Rutz et al. teach such methodswhereby iron powders are coated with a thermoplastic material. Thisplastic, in principle, is intended to act as a barrier between particlesto reduce induced eddy current losses. However, in addition to therelatively high cost of these thermoplastic coatings, there is aconsiderable further disadvantage to coating the iron powders withplastic. Specifically, plastic has poor mechanical strength compared tothe bulk alloy especially at high temperatures and has a tendency tocreep. Thus, as a result parts made using plastic-coated iron typicallyhave relatively low mechanical strength. Additionally, many of theseplastic-coated powders require a high level of binder when pressed. Thisresults in decreased density of the pressed core part and, consequently,a decrease in magnetic permeability and lower induction (B). Further,this material is normally pressed in a Hot Die resulting in a costly andcomplex manufacturing process.

Another major drawback exists with these thermoplastic-coated powders.The plastic coatings begin to degrade in the 150-200° C. range, andtypically melt or soften at temperatures in the 200-250° C. range. Thus,the applications in which parts made from iron particles coated withthermoplastics can be used are limited to near ambient temperature, lowstress applications for which dimensional control is not critical.Furthermore, it is generally not possible to achieve the stress/strainrelief benefits of high-temperature annealing i.e., annealing attemperatures in excess of about 150-300° C., with parts made usingthermoplastic-coated iron particles. These limitations and disadvantagesare also generally true for other known (typically polymeric) coatingsfor ferrous powders such as, for example, epoxies, phenolics, etc.

Hence, there is an important need in the industry for ferromagneticpowders to produce magnetic parts, particularly soft magnetic parts,that are well bonded (increased green strength, are tolerant of highertemperatures, have good mechanical properties, are configured inrelatively complex three-dimensional shapes and have low core loss.There is a particular need for such insulated powders, and parts madetherefrom, fabricated by a cost-effective method that provides precisedimensional control. Moreover, there is a need for such powders andparts made therefrom wherein the insulating properties of the coatingsdo not substantially degrade at relatively high temperatures. There isan additional need for processes for making these powders and the partsmade therefrom that results in a highly precise net shaped part.

SUMMARY OF THE INVENTION

Hence, the present invention provides ferromagnetic powder comprising aplurality of ferromagnetic particles having a diameter size of fromabout 40 to about 600 microns. The particles are coated with aninsulating coating comprised of from about 40% to about 85% (andpreferably from about 50%) by weight of FeO, Fe₃O₄, Fe₂O₃, (Fe₂O₃.H₂O)or combinations thereof and from about 15% to about 60% (and preferablyto about 50%) by weight of FePO₄, Fe₃(PO₄) ₂, FeHPO₄, FePO₄.2H₂O,Fe₃(PO₄)₂.8H₂O, FeCrO₄, FeMoO₄, FeC₂O₄, FeWO₄, or combinations thereof.The coating material preferably imparts an electrical insulation value,as determined between adjacent ferromagnetic particles, of at leastabout 1 milli-Ohm-cm.

The invention is further directed to ferromagnetic powder having acoating that permits adjacent particles to engage one another with aforce such that a part made by compressing the coated particles has anas pressed transverse rupture strength of at least about 8 Kpsi (and ashigh as 18-20 kpsi) as measured in accordance with MPIF Standard 41.Hence, parts made by compressing the ferromagnetic powder according tothe present invention have increased green strength as compared withparts made from uncoated powders.

In another embodiment, the ferromagnetic powder according to the presentinvention has an electrical insulation value as determined betweenadjacent coated particles that does not substantially degrade whensubjected to temperatures of greater than about 150° C. In yet anotherembodiment, the ferromagnetic powder has a coating that is substantiallyfree of organic materials.

In a further embodiment, the present invention is directed to anoxide-phosphate coating material for ferromagnetic particles. Thematerial comprises from about 50% to about 90% by weight of FeO, Fe₃O₄,Fe₂O₃, (Fe₂O₃.H₂O) or combinations thereof and from about 15% to about50% by weight of FePO₄, Fe₃(PO₄)₂, FeHPO₄, FePO₄.2H₂O, Fe₃(PO₄)₂.8H₂O,or combinations thereof. The coating permits adjacent particles toengage one another with a force such that a part made by compressing thecoated particles has a transverse rupture strength of at least about 8kpsi as measured in accordance with MPIF Standard 41.

The invention further pertains to a method of making ferromagneticpowder. The method according to the present invention involves providinga plurality of ferromagnetic particles and treating them with an aqueoussolution. The solution comprises from about 5 to about 50 grams perliter of a primary alkaline phosphate, an alkaline chromate, an alkalinetungstate, an alkaline molybdate, an alkaline oxalate or combinationsthereof, from about 0.1 to about 20 grams per liter of an oxidizingagent, and from about 0 to about 0.5 grams per liter of a wetting agent,a surfactant or both. The aqueous solution has a temperature of fromabout ambient to about 60° C. The treating step is performed for a timeperiod of from about 1 minute to about 20 minutes.

The invention is also directed to a method for making soft magneticparts from the coated particles and to the soft magnetic parts madetherefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the steps of an exemplary method ofmaking ferromagnetic powder according to the present invention;

FIG. 2 is a graph showing direct current characteristics of a part madein accordance with the present invention;

FIG. 3 are graphs showing permeability of a part made in accordance withthe present invention as a function of induction (FIG. 3a) and as afunction of applied field (FIG. 3b);

FIG. 4 is a graph showing core loss as a function of induction for apart made in accordance with the present invention; and

FIG. 5 is an optical micrograph of a cross section of a part madeaccording to the present invention shown at 1000×magnification.

FIG. 6 is an exploded view of a rotor according to the presentinvention.

FIG. 7 is a cross sectional view of a stator according to the presentinvention.

FIG. 8 is an exploded view of an armature assembly according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to ferromagnetic powder, a new coatingmaterial for the powder, soft and permanent magnetic parts madetherefrom and methods for manufacturing both the powders and the parts.As used herein, including in the claims, “magnetic parts” is intended tomean three-dimensional parts comprising ferromagnetic particles that arecompacted by the application of pressure thereto as for example in apowder metallurgy press or other suitable device. Such suitable devicesinclude, but are not limited to an extrusion press and a cold isostaticpress.

Ferromagnetic Powder

In one embodiment, the ferromagnetic powder of the present inventioncomprises ferromagnetic particles covered with a conversion coating. Theferromagnetic particles have an average size in the range of from about40 to about 600 microns, with the preferred range being from about 100to about 300 microns. The coating preferably has a thickness of fromabout 50 to about 5000 Å. In those instances where the powder is to beused in fabricating soft magnetic materials and parts, the coatingpreferably has a thickness of from about 50 to about 3000 Å.

Moreover, in those instances where the ferromagnetic powder according tothe present invention is for use in fabricating soft magnetic material,suitable ferromagnetic particles are particles of iron or iron alloyssuch as Fe—Si, Fe—Al, Fe—Si—Al, Fe—Ni, Fe—Co, Fe—Co—Ni, or combinationsthereof. Typically, alloys of iron have a higher permeability and lowercore losses when used in a magnetic circuit when compared with pureiron. However, pure iron functions satisfactorily and provides a higherinduction (high B), is softer, is easier to press to high density and isgenerally lower in cost. The particles may be any suitable particulatematerial, as for example, but not limited to, powders, fibers, wires andflakes with powders being preferred.

Additionally, when ferromagnetic powder according to the presentinvention is to be used in fabricating permanent magnet materials andparts, suitable particles are particles of carbon steel (0.9C, 1Mn),Tungsten steel (0.7C, 0.3Cr, 6W), 3.5% Cr Steel (0.9C, 0.35Cr), 15% CoSteel (1.9C, 7Cr, 0.5Mo, 15Co), KS Steel (0.9C, 3Cr, 4W, 35Co), MT Steel(2.0C, 8.0Al), Vicalloy (52Co, 14V), MK Steel (16Ni, 10Al, 12Co, 6Cu),Pt—Fe, iron powder (100Fe), FeCo (55Fe, 45Co), shock resisting toolsteel (0.5C, 1.40Mo, 3.25Cr) or combinations thereof. In such instances,the aforementioned coating should preferably have a thickness of fromabout 50 to about 1000 Å. This coating also provides lubricity to thepowders during pressing. Therefore the need to add an organic lubricantto the powder mass prior to pressing is effectively eliminated.

The ratio (weight) between the phosphate, molybdate, tungstate oroxalate component and the oxide component of the coating should beselected so as to influence the properties of the coating. For example,if the weight percentage of the phosphate, molybdate, tungstate oroxalate component is relatively high, then a poor bond between thecoating and the ferromagnetic particles may result. However, theinsulation value of the coating typically increases with increases insuch weight percentage. On the other hand, the ability of the coating tobond with the ferromagnetic particles increases as the weight percentageof the oxide increases. This improvement in bonding may occur at theexpense of the insulation value of the coating.

Hence, the coating disposed on each of the ferromagnetic particlesshould preferably comprise from about 40% to about 85% by weight andmost preferably from about 65% to about 80% by weight of either FeO,Fe₃O₄, Fe₂O₃, (Fe₂O₃.H₂O) or combinations thereof; and from about 15% toabout 60% by weight and most preferably from about 20% to about 35% byweight of FePO₄, Fe₃(PO₄)₂, FeHPO₄, FePO₄.2H₂O, Fe₃(PO₄)₂.8H₂O, FeCrO₄,FeMoO₄, FeC₂O₄, FeWO₄, and combinations thereof, with FePO₄Fe₃(PO₄)₂,FeHPO₄, FePO₄.2H₂O, Fe₃(PO₄)₂.8H₂O, and combinations of these beingpreferred. In those embodiments of the powder when an oxide/phosphatecoating is used as the coating on the ferromagnetic particles, theweight ratio is preferably selected so that the composition of thecoating approximates that of the mineral Vivianite (i.e.,Fe₃O₄+Fe₃(PO₄)₂−8H₂O) and hence comprises a “Vivianite-like” material.In a preferred embodiment, the coating is substantially free of organicmaterials.

The present invention should not however be construed as being limitedto ferromagnetic particles having a conversion coating of a specificcomposition disposed thereon. Rather, the mechanical and electricalinsulation properties, of the coating as described below, should directselection of coating composition.

The coating on the particles of the ferromagnetic powder of the presentinvention should preferably exhibit a number of properties. First, thecoating should be as thin as possible, consistent with the requirementthat the coating electrically insulate adjacent particles such that aninsulation value of about at least 1 to about 20 milli-Ohm-centimeter isachieved in a part fabricated therefrom, with higher values in, or evenabove, this range being preferred. The coating on each of theferromagnetic particles preferably has an electrical insulation value,as determined between adjacent particles, of at least about 1milli-Ohm-cm.

Thicknesses in the range of from about 1,000 to about 5,000 Å arepreferred for the coating when its electrical insulation value fallswithin the range identified above, with a thickness of about 2,000 Åbeing an especially preferred average thickness value.

The coating should preferably permit adjacent particles to bind togetherwith sufficient force that a part made by compacting the ferromagneticpowder of the present invention has sufficient transverse rupturestrength so that sintering after compaction is generally not required toobtain good mechanical properties. As used above, “sufficient transverserupture strength” should be construed as meaning a transverse rupturestrength in the range of from about at least 8 kpsi to about 20 kpsi,and preferably at least about 15 kpsi as determined in accordance withthe protocol of the American Society of Test Materials MPIF Standard 41.

The coating on the ferromagnetic powders according to the presentinvention should preferably exhibit lubricating properties, particularlyduring the initial stages of pressing operations when the coated powdersare used to fabricate soft magnetic parts. This lubricating featureshould optimally permit the particles to slip and slide by each otherduring pressing, thereby minimizing or eliminating point-to-pointwelding of the particles. As a result, a denser, and hence stronger,soft magnetic part is manufactured. Additionally, this lubricatingproperty facilitates part ejection from the dies thereby decreasingoverall manufacturing time and hence manufacturing cost.

The ferromagnetic powder according to the present invention preferablyhas an electrical insulation value that does not substantially degradewhen it is subjected to temperatures of greater than about 150° C.Hence, the coating is able to withstand relatively high temperatures,i.e. temperatures above about 150° C., without degrading. This permitsuse of magnetic parts made from the ferromagnetic powder of the presentinvention to be used in applications where soft magnetic parts made fromplastic-coated particles cannot be used due to degradation (typicallyabove about 150° C.) and melting (above about 200 to 250° C.) of thecoating. Examples of such applications include, but are not limited to,automotive parts such as stators, rotors, actuators, armatures,solenoids and motors used in the engine compartment of gasoline ordiesel motors. In addition, this high temperature tolerance permitsmagnetic parts made from the ferromagnetic powder of the presentinvention to be annealed at relatively high temperatures, i.e.temperatures in the 250 to 450° C. range, so as to reduce stress in theparts and consequently reduce core loss.

Additionally, the coating of the present ferromagnetic particles shouldpreferably be able to withstand relatively low temperatures, i.e.temperatures in the 20 to 200° K range. This characteristic permits suchparts to be used in cold operating environments, i.e., environments inthe −60° C. to 0° C. temperature range, without degradation orembrittlement of the coating. Examples of such environments are found incolder climates and jet airplanes.

The present invention is also directed to a coating material forferromagnetic particles. The coating material according to the presentinvention preferably comprises from about 50% to about 85% and mostpreferably, from about 65% to about 80% by weight of FeO, Fe₃O₄, Fe₂O₃,(Fe₂O₃.H₂O) or combinations thereof; and from about 15% to about 50% andmost preferably from about 20% to about 35% by weight of FePO₄,Fe₃(PO₄)₂, FeHPO₄, FePO₄.2H₂O, Fe₃(PO₄)₂.8H₂O, or combinations thereof.This coating is primarily an oxide in composition. In a preferredembodiment, the coating comprises a Vivianite-like material.

The coating according to this embodiment of the present inventionpreferably permits adjacent particles to engage one another with a forcesuch that a part made by compressing ferromagnetic particles having thecoating disposed thereon has an as pressed transverse rupture strengthof at least about 8 kpsi and most preferably greater than about 15 kpsi,as measured in accordance with MPIF Standard 41. Furthermore, thepresent coating preferably has an electrical insulation value of atleast about 200 micro-Ohm-cm, as determined between adjacentferromagnetic particles having said coating disposed thereon.Preferably, this electrical insulation value does not substantiallydegrade when subjected to temperatures of greater than about 150° C. Forpurposes of this invention substantially should be construed to mean notmore than about 5% at temperatures up to about 300° C. In an embodimentof the coating according to this invention, it is substantially free oforganic materials.

Method of Making Ferromagnetic Powder

In another embodiment, the present invention is directed to a method ofmaking ferromagnetic powders having the properties described above. Apreferred method of making ferromagnetic powder in accordance with thepresent invention comprises providing a plurality of ferromagneticparticles; and treating the particles with an aqueous solution. Theparticles preferably have a diameter of from about 40 to about 300microns. Examples of suitable ferromagnetic particles for use in thepresent invention method when used for making powders for soft magneticmaterials and parts, include, but are not limited to, particles of Fe,Fe—Si, Fe—Al, Fe—Si—Al, Fe—Ni, Fe—Co, Fe—Co—Ni, and combinationsthereof. Examples of suitable ferromagnetic particles for use in thepresent method when used for making powders for permanent magneticmaterials and parts, include, but are not limited to, particles of shockresisting tool steel (0.5C, 1.40Mo, 3.25Cr), carbon steel (0.9C, 1Mn),Tungsten steel (0.7C, 0.3Cr, 6W), 3.5% Cr Steel (0.9C, 0.35Cr), 15% CoSteel (1.9C, 7Cr, 0.5Mo, 15Co), KS Steel (0.9C, 3Cr, 4W, 35Co), MT Steel(2.0C, 8.0Al), Vicalloy (52Co, 14V), MK Steel (16Ni, 10Al, 12Co, 6Cu),Pt—Fe, iron powder (100Fe), FeCo (55Fe, 45Co) and combinations thereof.

A preferred aqueous solution for treating the ferromagnetic particlescomprises from about 1 to about 50 and preferably from about 10 to about20 grams per liter of a primary alkaline phosphate, an alkalinechromate, an alkaline tungstate, an alkaline molybdate, an alkalineoxalate or combinations thereof. Examples of primary alkaline phosphatessuitable for use in the present method include, but are not limited, toKH₂PO₄, NaH₂PO₄, NH₄H₂PO₄ and combinations thereof.

The aqueous solution for treating the ferromagnetic particles inaccordance with the present inventive method preferably comprises fromabout 0.1 to about 50 grams per liter of either an organic or aninorganic oxidizing agent. Examples of inorganic oxidizing agentssuitable for use in the present invention include, but are not limitedto, from about 0.3 to about 50, and preferably from about 0.5 to about 5grams per liter of KNO₃ or NaNO₃, from about 0.1 to about 50 andpreferably from about 5 to about 10 grams per liter of NaClO₃ or NaBrO₃,from about 0.1 to about 50 and preferably from about 0.1 to about 0.3grams per liter of KNO₂ or NaNO₂, from about 0.01 to about 0.1 andpreferably from about 0.03 to about 0.06 grams per liter H₂O₂.Additionally, from about 0.1 to about 2 grams per liter of hydroxylamineor hydroxylamine sulfate, from about 0.1 to about 2 grams per liter ofhydrazine and combinations thereof are suitable for use as acceleratorsof the present process.

Examples of organic oxidizing agents suitable for use in the presentinvention method include, but are not limited to, sodium m-nitrobenzene,nitrophenol, dinitrobenzene sulfonate, p-nitrobenzoic acid, nitrophenolnitroguanidine, nitrilloacetic acid and combinations thereof. Organicoxidizers are preferably used in an amount which is from about 0.3 toabout 10 and most preferably from about 0.5 to about 2.5 grams perliter. Alternatively (or additionally), phosphoric acid may be used inan amount which is from about 0.1 to about 5 grams per liter ofsolution.

In certain embodiments, the aqueous solution further comprises fromabout 0 to about 0.5 grams per liter and preferably from about 0.1 toabout 1 gram per liter of a wetting agent, a surfactant or both.Examples of surfactants preferred for use in the present method include,but are not limited to, sodium dodecyl benzyl sulfonate, lauryl sulfate,oxylated polyethers, ethoxylated polyethers and combinations thereof.

The aqueous solution should preferably have a temperature of from aboutambient to about 600° C. and most preferably from about 25° C. to about50° C. The treating step is preferably performed for a time period offrom about 1 minute to about 20 and most preferably from about 2 toabout 10 minutes. The aforementioned temperatures and time periods areexemplary only. Preferably, the time period is long enough to permit thepH of the aqueous solution to come to equilibrium. Such pH change ispreferably an overall increase of about 20%. The pH starting value ofthe solution depends on the detailed chemistry of the aqueous solution.However, in preferred aqueous solutions for use in the presentinvention, the starting value of the pH is from about 5 to about 6. Anexemplary pH change in the aqueous solution would for purposes of thepresent invention involve an increase from a starting pH of about 5.5 toand end point pH of from about 6.1 to about 6.5. Higher or lowertemperatures and pH's and longer or shorter time periods for treatingthe ferromagnetic particles are of course also within the scope of thepresent invention.

The method according to the present invention may further comprise thesteps of rinsing the particles to remove the aqueous solution and dryingthe particles. The process optionally comprises a chromate, molybdate ornitrate rinse to inhibit subsequent oxidation of the coated powders.

The method as described hereinabove can be summarized with reference toFIG. 1, wherein the first step of the method, identified at box 100, isproviding a plurality of ferromagnetic particles having an average sizein the range of 40 to 600 microns, with the preferred range being 60 to300 microns. As those skilled in the art will appreciate, the specificweight or volume of ferromagnetic particles provided in the first stepof the method will, of course, vary depending upon whether theferromagnetic powder is manufactured using a batch or a continuousprocess, and will depend upon the design of the equipment used to carryout the process. In examples of the method of making ferromagneticparticles provided below, an exemplary quantity of ferromagneticparticles is provided.

Optionally, as identified at box 102, the particles are cleaned in warmalkaline solution to remove any organic or surface contamination.Preferably, this cleaning step is carried out by immersing the particlesin the solution, although spraying and any other techniques forcontacting the particles with the cleaning solution under suitableconditions and for a suitable time to remove any unwanted contaminationcan also be used. An example of a suitable cleaning solution comprisesan aqueous solution of about 30 grams/liter NaOH, about 30 grams/literNa₂CO₃, about 30 grams/liter Na₃PO₄ and about 5 grams/liter Na₂SiO₃. Theoptional cleaning solution is preferably maintained at a temperature offrom about 90 to about 95° C., and the particles are preferably immersedin the solution for about 15 to about 30 minutes. If spraying or othertechniques are used to contact the particles with the cleaning solution,it is well within the skill of one of skill in the art to determine theappropriate duration for contacting the particles with the cleaningsolution. However, an exposure in the 1 to 10 minute range is generallysatisfactory. This cleaning step can further comprise decanting thecleaning solution and rinsing the thus cleaned particles in water havinga temperature of from about 50 to about 60° C. This rinsing step ispreferably performed several, e.g. three, times, using clean water foreach rinse cycle. Thereafter, one or more cold water rinses of theparticles is(are) performed, with the rinse water being decanted aftereach rinse and replaced with fresh water.

As the next step in the method, an optional acid dip (not shown) may becarried out. This optional step is carried out at ambient temperatures(i.e. from about 20 to about 25° C.) wherein the particles are subjectedto dilute acid at a concentration of about 0.1% to about 0.5% by weightfor a time period of about 3 minutes followed by a rinse (three times).This etching step is used to remove contamination, in particular sulfurcompounds from the surface. In the subsequent step as identified by box104, the particles are subjected to a solution that reacts with theparticles so as to create a conversion coating. The weight ratios andelectrical and mechanical properties of the coating, described above,are the key factors to be considered in selecting the solution andprocess parameters for creating the conversion coating on theferromagnetic particles.

Referring again to the step identified by box 104 in FIG. 1, a solutionsuitable for achieving the oxide/phosphate coating comprises ammoniadihydrogen phosphate, sodium nitrate, phosphoric acid or one or moreoxidizing agents. The ratio of these constituents of the solution, andthe process parameters used in the coating process are selected so thatfollowing reaction with the ferromagnetic particles a conversion coatinghaving the characteristics described above results.

At the step identified by box 104 in FIG. 1, the ferromagnetic particlesare subjected to the solution by preferably immersing the particles inthe same. Alternatively, the solution may be sprayed on the particles orbrought into contact with the particles using other known techniques.

Next, at the step identified by box 106, the solution is permitted toreact with the particles so as to form the conversion coating. Thespecific time for this reaction step will vary with the precisechemistry of the solution, the pH and temperature of the solution, andthe size of the particles used. However, a reaction time of from about 1to about 20 minutes, and most preferably from about 2 to about 10minutes, is typically sufficient. Preferably, the particles are agitatedand mixed during the reaction by known mechanical means to ensure asmany of the particles as possible react with the solution. The end pointof the reaction should preferably be determined to be the point at whichthe pH of the solution stops changing (i.e., reaches equilibrium). Withcontinuing reference to FIG. 1, as the next step in the process ofmaking the ferromagnetic powder of the present invention, identified bybox 108, the conversion coating solution is decanted.

The particles are then subjected to several rinse cycles to eliminateany solution remaining after the decanting step. The first rinse step,identified by box 110, involves subjecting the particles to hot waterhaving a temperature of about 50 to 60° for about 4 to 6 minutes.Preferably, the particles are immersed in hot water, but spray or otherknown techniques for applying the hot water may also be used. Theparticles are preferably agitated mechanically during the rinsing stepto enhance rinsing action. The hot water is then decanted. Preferably,this first rinse step is repeated once or twice.

The second rinse step, identified by box 112, is identical to the firstrinse step, except that cold water having a temperature of about 10 to20° C. is used. Thus, the particles are preferably immersed in the coldwater, but the latter may also be applied using spray or other knowntechniques. The rinse process preferably lasts about 4 to 6 minutes, andmechanical agitation is preferably applied during the process. The coldwater rinse is preferably repeated once or twice. Rinsing agents such asalcohol to reduce the surface tension of water may also be employed.Then, as identified by box 116, the ferromagnetic powder is dried. Apreferred method for drying in laboratory scale batches comprisesplacing powder in a large Buchner funnel and applying a vacuum theretofor from about 5 to about 10 minutes. Any known method for dryingpowdered materials can however be employed.

As an optional step prior to drying (box 114), the ferromagnetic powdermay be sealed to prevent rusting (oxidation). This sealing step may bedone using any known process for sealing powders such as chromating,etc.

EXAMPLE 1

One hundred (100) grams of substantially pure iron (99.87%) particles(Quebec Metal Powders, Atomet 590) having a mean particle size of about80 microns are cleaned by immersion into an aqueous solution of about 30grams/liter NaOH, about 30 grams/liter Na₂CO₃, about 30 grams/literNa₃PO₄ and about 5 grams/liter Na₂SiO₃ maintained at a temperature offrom about 90 to about 95° C. for a time period of about 20 minutes. Theclean particles are placed in a beaker and a solution containing 5 g/lof NH₄H₂PO₄, 0.3 g/l NaNO₃, 5 g/l NaNO₃ is added to the beaker so thatthe particles are completely immersed in the solution. The solution hasa pH of about 5.5 and is maintained at a temperature of about 40° C. Theparticles are then stirred continuously with a glass rod to ensure asmany of the particles as possible contact the solution. After about 2minutes of this immersion and stirring the solution is decanted.

Immediately thereafter, hot water having a temperature of about 55° C.is added to the beaker so that the particles are fully immersed. Theparticles remain immersed in the water for about 5 minutes, and areagitated to enhance rinsing action. The hot water is decanted and thishot water rinse step is repeated two times. Thereafter, cold water at atemperature of from about 10° C. to about 20° C. is added to the beakerso that the particles are immersed. Following about a 5 minute immersionwith agitation, the cold water is decanted. Then, this cold water rinsestep is repeated once.

The particles are then sealed with chromate by immersing them for about1 minute in a solution comprising 200 ppm of CrO₃ and 200 ppm of H₃PO₄in dionized water. The solution has a pH of about 4 and is maintained atroom temperature. The chromate solution is then decanted and theparticles are dried in the manner described above.

The oxide/phosphate-coated ferromagnetic powder made in accordance withthe process described in Example 1 is analyzed as follows. First, thecoating on 5 samples of the powder from Example 1 is analyzed by EnergyDispersive X-Ray Spectrometer (EDAX) to determine the composition of thecoating. The results are as follows as reported in Table 1.

TABLE 1 Element Weight Percent Atomic Percent Sample O P Fe Total O P Fe1  7.37 1.21 96.15 104.73 20.74 1.76 77.51 2 16.26 4.00 84.46 104.7938.34 4.85 56.80 3 11.95 2.86 92.08 106.88 30.02 3.71 66.28 4 18.56 4.3081.48 104.34 42.06 5.03 52.91 5 16.26 4.08 81.87 102.21 38.87 5.04 56.09

The powder is then pressed at 60 tons/in² into a torrous which wasmeasured by an AC magnetic hysteresis instrument and is determined tohave a maximum inductance-related to coating thickness of about 12.3kGauss at 40 Oersted applied field.

To measure the binding strength between adjacent particles, a softmagnetic part having the shape of a bar is made using the ferromagneticpowder from Example 1 in accordance with MPIF Standard 41. Thetransverse rupture strength of the bar is determined without anyfollow-on processes such as annealing or sintering, also in accordancewith this MPIF standard. The part was determined to have a transverserupture strength of 18,000 pounds/square inch.

To determine the ability of the oxide/phosphate coating to withstanddegradation in temperatures above 150° C. and below −50° C., additionalsoft magnetic parts made using ferromagnetic powder from Example 1 arefabricated in accordance with MPIF Standard 41. Some of these parts arethen annealed at a temperature reaching 400° C. and others are subjectedto temperatures reaching −70° C. Subsequent analysis of the parts inaccordance with MPIF Standard 41 reveal no decrease in transverserupture strength. In addition, other analyses indicates an improvementin permeability, saturation induction and core loss of the part afterthe anneal. Further testing at 300° C. is done and no degradation intransverse rupture strength is measured.

Method of Making Soft Magnetic Parts

The present invention is further directed to a method of making softmagnetic parts. As the first step in this method, a source of theferromagnetic powder of the present invention is provided. This powdermay be obtained using the method of making ferromagnetic powder asdescribed above or by using other methods, the only requirement beingthat the ferromagnetic powder have the properties described above. Theplurality of ferromagnetic particles are coated with a coating thatpermits adjacent particles to engage one another with a force such theresultant part has an as pressed transverse rupture strength of at least8 kpsi, as measured in accordance with MPIF Standard 41. Preferably thecoating is insulating and its electrical insulation value does notdegrade at temperatures over 150° C.

Each of the particles in the part is preferably coated with a materialcomprising from about 40% to about 85% by weight of FeO, Fe₃O₄, Fe₂O₃,(Fe₂O₃.H₂O) or combinations thereof; and from about 15% to about 60% byweight of FePO₄, Fe₃(PO₄)₂, FeHPO₄, FePO₄.2H₂O, Fe₃(PO₄)₂.8H₂O, FeCrO₄,FeMoO₄, FeC₂O₄, FeWO₄, or combinations thereof. The coating materialpreferably comprises an oxide and a phosphate conversion coating. Inthis preferred coating, the oxide and phosphate preferably have a weightratio of from about 2 parts oxide to about 4 parts oxide to about onepart phosphate. The coating is most preferably a Vivianite-likematerial. In some embodiments, the coating is substantially free oforganic materials.

Preferably, the coating step in the method for making the soft magneticparts is comprised of treating the particles with an aqueous solutioncomprising from about 5 to about 50 grams per liter of a primaryalkaline phosphate, an alkaline chromate, an alkaline tungstate, analkaline molybdate, an alkaline oxalate or combinations thereof. Thesolution further comprises from about 0.1 to about 20 grams per liter ofan oxidizing agent, and from about 0 to about 0.5 grams per liter of awetting agent, a surfactant or both. The aqueous solution shouldpreferably be maintained at a temperature of from about 30° C. to about60° C., and the treatment step should be carried out for a time periodof from about 1 minute to about 20 minutes.

The coated particles are consolidated by uni-axial pressing into a part.This step preferably comprises compacting the ferromagnetic powder to adensity approximating “full density”, i.e., the density at which thecoated particles making up the part have at least non-interconnectedporosity and preferably no porosity. Hence, the as pressed density ofthis part is from about 7.4 to about 7.6 g/cm³.

This compacting step is preferably effected with powder dies andpresses, both traditional and non-traditional. However, other techniquesmay also be satisfactorily employed to compact the coated particles.These techniques include, but are not limited to, high velocityprojection (similar to thermal spraying), roll-bonding, hot isostaticpressing (hipping), cold isostatic pressing (cipping), forging, powderextruding, coining or rolling the ferromagnetic powder. In instanceswhere the ferromagnetic powder is compacted using a conventional die anduni-axial powder press, a preferred pressure for obtaining the desireddensities is from about 25 tons/square inch to about 60 tons/squareinch. The step of compacting is preferably done at room temperature.

An important advantage of soft magnetic parts made in accordance withthe present invention is that high-temperature sintering of the part isgenerally not required after compaction in order to obtain desireddensities and mechanical properties in the part. Hence, the presentinvention is also directed to a method of making parts having increasedgreen strength as pressed).

Hence, the part is removed from the die and following removal of thesoft magnetic part from the press or other apparatus, it may bedesirable to subject the part to a low temperature anneal to reduceinternal stress (coercive strain). This optional annealing step alsoserves to improve the magnetic properties of the resultant part. Thisoccurs because the large stresses induced by compacting the powders inthe die typically increase the coercive force H_(c) of the part. Theseincreases in H_(c) may therefore result in increases in core losses inthe part to a level which may or may not be acceptable depending uponthe intended operating temperature and application frequency of use. Alow-temperature anneal is typically carried out by placing the part inan oven in a non oxidizing environment and gradually heating.Alternatively, coercive strain in the part may be reduced by any otherknown methods for doing so.

EXAMPLE 2

Compaction

In an exemplary method of making a soft magnetic part in accordance withthe present invention, nine (9) grams of the ferromagnetic powder of thepresent invention made in accordance with the process of Example 1,described above, is charged into a 1″ diameter, 0.8″ interior diametertorroidal die mounted in a uni-axial 50 ton hydraulic press (Dake 50H)and is compacted at a pressure of 60 tons/square inch. The resultantpressed torrous is removed from the die and then subjected to alow-temperature anneal for 30 minutes at a temperature of 250° C. to300° C. in a non-oxidizing environment at a ramp rate of less than3°/second.

Magnetic Analysis

Two coils are wound on the torrous using 24 gauge insulated coppertransformer wire. Each coil has 50 turns of wire, tightly wound throughthe center of the annulus. A current of 8 amps is applied through thefirst coil. The second coil is connected to a 16-bit A/D converter andthen to a computer to record data. The current is applied by agalvanostat having a frequency response that is not dependent onfrequency at frequencies up to about 18 KHz. The current waveform isdetermined by a voltage waveform supplied by a function generatorgalvanostat. This device and associated software is known as a magnetichysteresis instrument. The waveform is a sine wave applied atfrequencies that vary from about 1 Hz to about 600 Hz. The results ofthis analysis are as follows:

Saturation Induction

Direct current characteristic is shown in FIG. 2 wherein applied fieldis varied from 0 to 43 Oersted and induction recorded by a magnetichysteresis instrument.

Permeability

Permeability as a function of induction is shown in FIG. 3a andinstantaneous permeability (dB/dH) as a function of applied field inFIG. 3b. Note that for specimens measured at 200 Hz, maximum valueoccurs at H_(c).

Core Loss (Watts/Pound)

FIG. 4 shows core loss as a function of induction for the part. Coreloss is measured by integrating B v. H curve and is illustrated atfrequencies of 60, 120, 300 and 900 Hz.

Mechanical Properties

A second specimen is made in the shape of a bar by placing 18 grams ofpowder made according to Example 1 into a rectangular die and pressingthe powder at 60 kpsi. The specimen is then tested according to MPIFtesting protocols. The results of this analysis are shown as follows inTable 2:

TABLE 2 Nominal Transverse Rupture Stress (MPIF Standard 41) 18 kpsiTensile Stress (ASTM E8) 60 kpsi Rockwell Hardness B scale (ASTM A370)R_(B) 75 Vickers Microhardness (ASTM E384 50 grams) 260 Kgf/mm²

Soft Magnetic Part

The present invention is further directed to a soft magnetic partcomprising a three-dimensional structure. The structure is comprised ofconsolidated ferromagnetic particles having a coating of a materialhaving an electrical insulation value that does not degrade attemperatures above 150° C. The magnetic part according to the presentinvention preferably has a transverse rupture strength as determined inaccordance with MPIF Standard 41, of at least about 8 kpsi and mostpreferably from about 12 kpsi to about 20 kpsi. Preferably, a softmagnetic part according to the present invention has an electricalinsulation value which is at least about 1 milli-Ohm-cm, as determinedbetween adjacent ones of consolidated coated ferromagnetic particles.

A soft magnetic part according to the invention comprises athree-dimensional structure of consolidated ferromagnetic particlescoated with a conversion coating material. The material preferablycomprises from about 2 to about 4 parts by weight of an oxide to onepart of a chromate, molybdate, oxalate, phosphate, tungstate or acombinations thereof. The coating may be substantially free of organicmaterials.

The present invention also includes a stator for an alternating currentgenerator. It should be understood that the present invention should notbe construed as to be limited to a soft magnetic part having the shapeof a stator. Instead the invention should be construed to include otherferromagnetic parts having their own respective three dimensionalshapes. Hence, the present invention magnetic part includes all magneticmotor and generator parts, armatures, rotors, solenoids, linearactuators, gears, ignition cores, transformers (feedback, horizontalflyback, power conditioning, ferroresonant), ignition coils, converters,inverters and the like.

A stator in accordance with the present invention comprises a pluralityof ferromagnetic particles that are consolidated in the shape of astator core. A preferred shape for the stator core can be seen in FIG. 7wherein annular yoke 2 has a plurality of integral innercircumferentially spaced projections 3 radiating and extending inwardlyand defining slots 5. Each of the ferromagnetic particles has a coatingof a material that has an electrical insulation value that does notdegrade at temperatures above 150° C. The core has an as pressedtransverse rupture strength, as determined in accordance with MPIFStandard 41, of at least 8 kpsi. In a stator according to the presentinvention, the electrical insulation value is preferably at least about1 milli-Ohm-cm, as determined between adjacent consolidated coatedferromagnetic particles.

EXAMPLE 3

An exemplary stator for an alternating current generator according tothe present invention is made using a powder die mounted in a 220 tonhydraulic press (Cincinnati 220). The die has a cavity with aconfiguration corresponding to that of the stator. More particularly,the die has an annular cavity with a plurality of slots communicatingwith the cavity and extending radially inwardly from the cavity. Theferromagnetic powder of the present invention made in accordance withthe process used for Example 1, described above, is charged into the dieand is compacted at a pressure of 30 tons/square inch. The resultantpressed stator is removed from the die. The stator is then subjected toa low-temperature anneal. (250 to 300° C., <3°/second for 30 minutes inan inert atmosphere).

Magnetic Analysis

Coils of wire are wound on each of the radially inwardly extending“fingers” or poles of the stator using 24 gauge insulated coppertransformer wire. Each coil has 25 turns of wire, tightly wound aroundeach finger. A current of 0.25 amps (300 Hz) is applied through thefirst coil and a hall probe placed at the end of the pole and the fieldvalue recorded as 30 Gauss.

Transverse Rupture Strength Analysis

A second stator is made as described immediately above and a rectangularsection is mechanically sawed out and removed. This specimen is testedfor transverse rupture strength (MPIF Standard 41). The results of thisanalysis indicate the stator has a transverse rupture strength of 18kpsi.

The rectangular cross section is polished metallo-graphically and anoptical micrograph is taken and shown in FIG. 5 wherein powder sizedistribution is shown. No porosity is apparent in the cross section anda continuous coating is shown surrounding each individual particle. Thecoating thickness appears to be less than 1 micron.

A rotor in accordance with the present invention is shown in FIG. 6wherein a plurality of ferromagnetic particles according to the presentinvention are consolidated in the shape of annular shaped cylinder 2defining cylindrical void 4 through which passes elongated cylindricalshaft 6 for rotating the rotor. Hollow cylinder casing 8 encaseselongated annular shaped cylinder 2 and is typically comprised ofcompressed permanent magnetic particles which can be pure iron or coatedparticles.

The invention further includes an armature assembly for an alternatingcurrent generator comprised of a plurality of ferromagnetic particlesconsolidated in the shape of an armature core and a shaft for rotatingthe armature thereon. The shape comprises elongated annular shapedcylinder 2 defining elongated cylindrical void 4 through which passesshaft 6. Annular shaped cylinder 2 has a plurality of troughs 8 atspaced intervals extending lengthwise along exterior surface 9 thereof.

Of course the shapes of the aforementioned parts are exemplary only andshould be construed to include the shape of all and any magnetic partsmade from compressed particles according to the present invention.

Since certain changes may be made in the ferromagnetic powder, coating,soft (and hard) magnetic parts, and methods of making the same,described above without departing from the spirit and scope of thepresent invention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings be interpreted in anillustrative and not in a limiting sense.

What is claimed is:
 1. A method of making a soft magnetic partcomprising the steps of: a. providing a plurality of ferromagneticparticles; b. applying a coating composition onto each of saidparticles, said coating composition comprising from about 40% to about85% by weight of a member selected from the group consisting of FeO,Fe₃O₄, Fe₂O₃, (Fe₂O₃.H₂O) and combinations thereof; and from about 15%to about 60% by weight of a member selected from the group consisting ofFePO₄, Fe₃(PO₄)₂, FeHPO₄, Fe₃ (PO₄)₂.2H₂O, Fe₂(PO₄)₃.8H₂O, FeCrO₄,FeMoO₄, FeC₂O₄, FeWo₄, and combinations thereof; and c. consolidatingsaid coated particles in the shape of said part.
 2. A method accordingto claim 1, wherein the composition of said coating is approximatelyFe₃O₄+Fe₃(PO₄)₂−8H₂O.
 3. A method according to claim 1, furthercomprising the step of: d. annealing said soft magnetic part.
 4. Amethod according to claim 3, wherein at least a portion of saidannealing step is performed at temperatures in excess of about 150° C.5. A method according to claim 4, wherein at least a portion of saidannealing step is performed at temperatures in excess of about 200° C.6. A method according to claim 1, wherein said applying step iscomprised of treating said particles with an aqueous solution comprisingfrom about 5 to about 50 grams per liter of a member selected from thegroup consisting of a primary alkaline phosphate, an alkaline chromate,an alkaline tungstate, an alkaline molybdate, an alkaline oxalate andcombinations thereof; from about 0.1 to about 50 grams per liter of anoxidizing agent; and from about 0 to about 0.5 grams per liter of amember of the group consisting of a wetting agent, a surfactant andcombinations thereof; said aqueous solution being at a temperature offrom about 30° C. to about 60° C., and said step of treating beingcarried out for a time period of from about 1 minute to about 20minutes.
 7. A method of making a soft magnetic part comprising the stepsof: a. providing a plurality of ferromagnetic particles; and b. applyinga coating composition onto said particles, said coating compositioncomprising a conversion coating that permits adjacent particles toengage one another with a force such that said part made from saidferromagnetic particles having said coating applied thereon has an aspressed transverse rupture strength of at least 8 Kpsi and which has anelectrical insulation value that does not degrade at temperatures over150° C.; and c. consolidating said coated particles in the shape of saidpart.
 8. A method of making a soft magnetic part comprising the stepsof: a. providing a plurality of ferromagnetic particles; b. applying acoating composition onto each of said particles, said coatingcomposition comprising a material comprising an oxide and a phosphateconversion coating, said oxide and phosphate in a weight ratio of fromabout 2 parts oxide to about 4 parts oxide to about one part phosphate,wherein said coating is substantially free of organic materials; and c.consolidating said coated particles in the shape of said part.
 9. Amethod according to claim 8, annealing said part at a temperature above150° C.